The antics of the upholders of the climate orthodoxy are becoming truly hilarious. In response to Nic Lewis's findings about the contradictions and failures of the draft IPCC report, we have now had no fewer than three "rebuttals" (Desmog, Media Matters, Think Progress) none of which link to the actual article. In fact none of them even mentions Lewis, preferring instead to concentrate on Matt Ridley's WSJ op-ed.

It does rather tell a story.

I think we can say that there is a consensus that the IPCC's models don't include their latest best estimates of aerosol forcings and that the empirical evidence suggests that that climate sensitivity is low.

There have been some scientific objections to Lewis's paper however and it's worth pointing these out.

Lewis references this claim to AR4 WG1 Box 10.2, and I can't find anything like that in the referenced text.

And on Twitter, we have the observation that Aldrin et al also considered a scenario in which the cloud lifetime effect was considered and that this raised the sensitivity. The tweet says it raised it to 3.3, but this is the mean rather than the most-likely value, which is still around the 2 mark. The cloud lifetime effect is probably worthy of a blog post in its own right.

That said, these last two observations seem at least to be addressing Lewis's arguments and are worthy of follow-up.

Reader Comments (11)

The referenced text does state this:

"Two recent studies use a modelled relation between climate sensitivity and tropical SSTs in the Last Glacial Maximum (LGM) and proxy records of the latter to estimate ranges of climate sensitivity (Annan et al., 2005b; Schneider von Deimling et al., 2006; see Section 9.6). While both of these estimates overlap with results from the instrumental period and results from other AOGCMS, the results differ substantially due to different forcings and the different relationships between LGM SSTs and sensitivity in the models used. Therefore, LGM proxy data provide support for the range of climate sensitivity based on other lines of evidence. "

I think it is interesting to note the difference in the response from the independent thinkers - Curry and Betts spring to mind - and the gatekeepers of the ideology

I would observe that the unscientific response of the ideologues simply breeds scepticism. As I have commented before these people are too thick to see this! Curry and Betts are actually doing a better job.

Of course, I approach this purely from the point of view of wanting the science to re-establish itself which clearly puts some distance between me and the ideologues.

The unfortunate thing is that the ideologues have been calling the shots, aided and abetted by a compliant MSM, for far too long. Kudos to Judith Curry for being the first (and possibly only) true scientists to try to redress the balance. Whilst Richard Betts shows willing he is apparently still wedded to the fanciful notion that the output of computer models is science. Until he can get over that disability I cannot regard him as a true friend of science.

Everyone's waiting for the babies at skepticalscience to read Lewis' objections, and write their 'rebuttal' so they can refer to it. All that is needed is for something to be said in response. You'll see everyone picking it up.

When Aldrin et al. (2012) adds a cloud lifetime (2nd indirect) effect of -0.25 W/m^2 as well as his direct and (1st) indirect aerosol forcing, the mode of his ECS PDF increases from 1.6 to 1.8. Increasing the addition to -0.5 causes a similar further increase. The mean goes up a lot (to about 2.45 C and 3.2 C per his Figure 6 h) and i), on my measurements) because the tail becomes much fatter - a reflection of the distorting effect of using a uniform prior for ECS. But, given the revised aerosol forcing estimates given in the AR5 WG1 SOD, I can see no justification at all for increasing the aerosol indirect forcing by adding either -0.25 or -0.5 W/m^2. On the contrary, it should be reduced by adding something like +0.5 W/m^2 to be consistent with the SOD estimates.

What is much more relevant and interesting is that although Aldrin states that for its main results, it uses the AR4 forcings, including for aerosols (-0.5 [± 0.4] W/m^2 direct , -0.7 [-1.1, +0.4] W/m^2 indirect), its posterior means are in line with the much lower AR5 purely observational estimates: -0.4 W/m^2 for direct (Fig. 12) and -0.3 W/m^2 for indirect (Fig. 15) aerosol forcing. That provides additional support for the accuracy of the AR5 purely-observational total aerosol adjusted forcing estimate of -0.73 W/m^2, , which is what I used for my climate sensitivity estimate.

It is a bit surprising that adding cloud lifetime effect forcing makes much difference, insofar as Aldrin is estimating aerosol forcing as part of his Bayesian procedure. The reason it does have an effect may, I think, be because he isn't using wide enough priors for direct and (1st) indirect aerosol forcing for the posterior mean fully to reflect what the model-observational data comparison is implying. His prior distributions for direct/indirect aerosol forcings are normal/lognormal distributions matching the AR4 uncertainty ranges, and when extra forcing of -0.25 or -0.5 W/m^2 is added his prior mean total aerosol forcing is very substantially more negative than per AR5, so that the data is fighting against the tail of the priors. It may possibly also be because his MCMC (Markov Chain Monte Carlo) simulation has a barrier at zero for (1st) indirect aerosol forcing, preventing any part of the posterior distribution going positive to counteract the added -0.25/-0.5 W/m^2 forcing (his Fig. 15 IndirAero graph does not extend to positive values, although it may be just the graph that is cut off).

The main result from Aldrin et al., as reported by Aldrin et al., is an ECS of 2.0°C. The authors caution that this result probably isn't an apples to apples comparison to other ECS estimates due to the unaccounted for cloud term, and find that the value increases to ~2.5-3.3°C with clouds.

Rather than report either of these values, you simply claim Aldrin et al. "an impressively thorough study, gives a most likely estimate for ECS of 1.6°C...".

Ridley likewse claims, "An impressive study published this year by Magne Aldrin of the Norwegian Computing Center and colleagues gives a most-likely estimate of 1.6°C."

[Snip - keep to the science please]

Claiming that ECS cannot be estimated by paleo data is absurd, especially when so many are aware of efforts like the PALAEOSENS project and various paleoclimatic intercomparison groups.

[Snip - keep to the science please]

I hope you take this criticism in the constructive context in which it is being offered.

That is bizarre. BH snips half of the post from 'thingsbreak' because it is not "science" and yet allows others to spout non-science at will. The original un-snipped text of the post, over at thingsbreak, is inoffensive in the extreme - why snip so much of it?